Again, crayfishes are found in the fresh waters of the Southern hemisphere; and almost the whole of what has been said respecting the structure of the English crayfish applies to these; in other words, their general plan is the same. But, in these southern crayfishes, the podobranchiæ have no distinct lamina, and the first somite of the abdomen is devoid of appendages in both sexes. The southern crayfishes, like those of the Northern hemisphere, are divisible into many species; and these species {250} are susceptible of being grouped into six genera—_Astacoides_ (fig. 65), _Astacopsis_, _Chæraps_, _Parastacus_ (fig. 64), _Engæus_, and _Paranephrops_—on the same principle as that which has led to the grouping of the Northern forms into two genera. But the same convenience which has {252} led to the association of groups of similar species into genera, has given rise to the combination of allied genera into higher groups, which are termed _Families_. It is obvious that the definition of a family, as a statement of the characters in which a certain number of genera agree, is another morphological abstraction, which stands in the same relation to generic, as generic do to specific abstractions. Moreover, the definition of the family is a statement of the plan of all the genera comprised in that family. [Illustration: FIG. 65.—_Astacoides madagascarensis_ (2/3 nat. size). From Madagascar.] The family of the Northern crayfishes is termed _Potamobiidæ_; that of the Southern crayfishes, _Parastacidæ_. But these two families have in common all those structural characters which are special to neither; and, carrying out the metaphorical nomenclature of the zoologist a stage further, we may say that the two form a _Tribe_—the definition of which describes the plan which is common to both families. [Illustration: FIG. 66.—Diagram of the morphological relations of the Astacina.] It may conduce to intelligibility if these results are put into a graphic form. In fig. 66, A. is a diagram representing the plan of an animal in which all the externally visible parts which are found, more or less modified, in the natural objects which we call individual crayfishes are roughly sketched. It represents the plan of the tribe. B. is a diagram exhibiting such a modification of A. as converts it into the plan common to the whole family of the _Parastacidæ_. C. stands in the same relation to the _Potamobiidæ_. If the scheme were thoroughly worked out, diagrams representing the peculiarities of {254} form which characterize each of the genera and species, would appear in the place of the names of the former, or of the circles which represent the latter. All these figures would represent abstractions—mental images which have no existence outside the mind. Actual facts would begin with drawings of individual animals, which we may suppose to occupy the place of the dots above the upper line in the diagram.